Technical Field
The present invention relates to partial discharge detection systems and, particularly, to systems for detecting partial discharge pulses synchronized with an electrical supply voltage.
Description of the Related Art
Partial discharge detection is particularly used for identifying and measuring partial discharges in electrical components and apparatus, such as: medium, high or extra-high voltage cables, cable joints, overhead line insulators, medium and high voltage switchboard boxes, high and extra-high voltage cables using GIS (Gas Insulated Switchgear), substations, transformers, motors.
The term partial discharges is intended to indicate an undesired recombination of electric charges occurring in the dielectric (insulating) material of electric components, when the latter have defects of various types, eventually leading to dielectric destruction. Here, a pulse current is generated in portions of dielectric material and causes an electromagnetic wave to propagate through the power or ground cables of the relevant electric system, and radiating through the various surrounding media (dielectric material, metals, air, etc.).
For executing partial discharge measurements on AC (Alternate Current) electrical components it is important to have a phase reference signal, i.e. a signal that is synchronized in phase and frequency with the AC voltage powering the electrical component. Useful diagnostic figures are obtained by plotting the maximum amplitude of the partial discharge pulses versus the phase of the supply voltage when they occurred.
In some cases, sensing the AC supply voltage to obtain its phase angle implies using specific sensors that have to be connected to the components under test. This operation in general requires the component to be disconnected from its supply (shut off) and then reconnected: this operation is often impractical, has high indirect costs, and cannot be done at all in many cases.
Document WO-A-2009-150627 describes, inter alia, a partial discharge detection device of small size, totally insulated and self-powered, which allows measurements to be performed with the highest safety with no need for direct connection to the system under examination. The device comprises a wide-band antenna adapted to act as an electric field sensor and including a first planar conductor (i.e. a ground plane) cooperating with a second conductor whose profile converges towards the first planar conductor at one point or one line. This partial discharge detection device can also detect a synchronization signal, which is obtained by picking up the supply voltage of the discharge generating components.
There are practical conditions in which the detection of the supply voltage of a partial discharge generating component can be performed neither in contact nor in contactless technologies on the partial discharge generating component but it has to be performed on another electrical component and remotely from the component under test.
Document JP-A-6-11534 discloses a partial discharge measuring system comprising a solenoid coil detection part which is provided at a power cable which is laid in a ductwork inside an underground manhole, the output signal is detected by a partial discharge detection part and then the detection signal is transmitted to the antenna of a manhole lid by a detection signal transmission part. A DC regulation power supply receives power from the cable by a transformer for receiving power supply. The applied voltage phase information of the cable is transmitted on the electric wave of a mobile telephone from a transmission terminal side substation provided with a voltage transformer. A partial voltage and applied voltage phase reception device which is provided near the manhole lid is provided with a radio signal reception part and a telephone signal reception antenna, obtains the partial discharge signal of the power cable under test and the applied voltage phase information signal, and then analyses the partial discharge pulse with the applied voltage phase as parameters.
Document JP200307551 describes a technique according to which a radio wave having a time signal sent by a GPS satellite is received at a partial discharge signal detecting part and at an application voltage signal detecting part. This time signal and the signals detected by the application voltage signal detecting part and by the partial discharge signal detecting part are recorded.